flat steel, not angle iron. At least 1" height, at least 1/8" thick (across). More is Better. Paint it to prevent rusting.
If you can get a tube of 1/2" by 1" (with tube walls only 1/16th" thick), that is good too, and it looks good because of the rounded corners on a factory made smooth tube. Any other way to round out the corners is made by a handheld grinder, or with a belt sander, or some other tool, and may give fairly good results. A tube also have the advantage that it is easy to get a screw through it and into the plywood.
The steel has to go underneath the 15" piece sticking out for at least 12", and then back inside underneath the counter for at least 18". More is Better. Screws into the ply all the way through and then barely into the silestone.
How many plies (layers) does the plywood have? More is Better.
This is just my assessment, based on my experience with steel. An angle iron is not stronger than a flat bar in this case. A U channel is better, and the best is a tube or a solid 1/4" or 1/2" thick. The height is the most important dimension, not the thickness. That is why I said "flat" -- Now, you are probably wondering how to screw a flat bar... and for that, you would have to weld a few pieces of a 1/2" flat bar, or an entire flat bar onto one side. In other words, the only advantage to an angle iron is its screwability.
david